Although a cathode ray tube (CRT, also generally referred to as “Braun tube”) has been a mainstream of display devices, liquid crystal displays have been used more and more, because they have less thickness and weight than CRT and can produce images with better quality.
Demands have been increasingly made to provide better color reproduction and higher contrast ratio with broadening applications of liquid crystal displays as monitors for desktop personal computers, monitors for printing or designing, and liquid crystal televisions. In particular, viewing angle characteristics in color reproduction and contrast ratio are very important in liquid crystal televisions which become popular after the beginning of high-definition television (HDTV) broadcasting. For example, to view a flat liquid crystal television at a corner of a living room at home, the flat liquid crystal television must have contrast ratio and chromaticity not changing at viewing angles within ±45 degrees.
The viewing angle characteristics of liquid crystal display apparatuses are derived from the viewing angle characteristics of polarizers comprising an oriented component of iodine or a dichroic dye, and the viewing angle characteristics of liquid crystal layers. As a possible solution to improve the viewing angle characteristics, an optical film showing a phase difference is generally used. As another possible solution, the techniques disclosed in PCT Japanese Translation Patent Publications No. 2001-504328 and No. 2003-532141 use an E-polarizer showing satisfactory characteristics at wide viewing angles.
Liquid crystal display apparatuses which give a display by the action of polarization have an inherent issue that their characteristics such as contrast ratio and chromaticity significantly vary depending on the viewing angle, due to anisotropy of polarizers and liquid crystal molecules. To solve this issue, an attempt has been made to compensate the viewing angle characteristics using a substantially transparent optical film having uniaxial or biaxial refractive anisotropy. This technique has been employed in most of liquid crystal display apparatuses typically used as liquid crystal televisions. According to this technique, however, compensation cannot be achieved homogenously at visible wavelengths, because the liquid crystal and optical film both have refractive-anisotropic chromatic dispersion (wavelength dispersion).
Taking a three-primary color display system as an example, a blue display, a green display, and a red display are produced by transmitted light at wavelengths of 420 to 490 nm, at wavelengths of 520 to 570 nm, and at wavelengths of 610 to 650 nm, respectively, while depending on the wavelengths of emitted light of a light source to be used. If a high priority is given to the viewing angle compensation in contrast ratio, the compensation is preferably optimized at wavelengths of about 550 nm at which the spectral luminous efficacy is high. Accordingly, the compensation is not sufficient at shorter wavelengths for producing a blue display and at longer wavelengths for producing a red display. This causes insufficient compensation of viewing angle in blue and red displays and invites problems such as coloring in a black display. If the compensation is optimized at wavelengths for a blue display so as to avoid the coloring, the viewing angle compensation at wavelengths for a red display further becomes out of optimum conditions, and thereby the coloring in a red display becomes more intensive. In addition, the contrast ratio varies more significantly depending on the viewing angle, because the optimization is not achieved at a wavelength of 550 nm at which the spectral luminous efficacy is high. In contrast, if the viewing angle compensation is optimized at wavelengths for a red display, coloring in a blue display and the dependency of the contrast ratio on the viewing angle become more significant.
As another possible solution, a film showing reciprocal dispersion is used as an optical film for compensation. The film has refractive-anisotropic wavelength dispersion characteristics that decreases with a decreasing wavelength, which is opposite to regular optical film. This technique, however, has a narrow margin in design of materials for the optical film. Additionally, it cannot easily compensate large screens as in liquid crystal televisions, because the compensation is out of optimum conditions when the thickness of the liquid crystal layer locally varies as in such large screens.
In the above-mentioned technique, an E-polarizer having a disc-form molecular structure is configured to have a transmission axis along with an extraordinary wave axis and is used as a polarizing layer having wide viewing angle characteristics. According to this technique, however, the polarizing layer has a lower dichroic ratio and thereby shows a lower front contrast ratio than a polarizer comprising iodine or a rod-like dichroic dye oriented and having an absorption axis along with the extraordinary wave axis.